Acrylic resins such as polymethyl methacrylate have been used for optical lenses, digital audio disks, optical memory disks, optical fibers, etc. by virtue of their properties such as transparency and strength (Japanese Unexamined Patent Publications No. 74701/1982 and No. 180906/1988. However, with conventional acrylic resins, drawbacks have been pointed out such that their glass transition temperature is low, and their water absorptivity is high. Various methods have been proposed to solve these problems. For example, Japanese Unexamined Patent Publications No. 151413/1987 and No. 72708/1988 and Japanese Examined Patent Publication No. 28419/1971 disclose methods intended to improve the heat resistance or to reduce the after absorptivity. However, these methods still have many problems with respect to e.g. the moldability and have not been practically used.
As opposed to such acrylic resins, epoxy resins have been known as resins having high heat resistance. However, epoxy resins have a problem with respect to e.g. the molding time, although they are excellent in the optical properties such as transparency and optical anisotropy like the acrylic resins. In the bulk reaction by photocuring by means of ultraviolet rays, a problem has been pointed out such that it is difficult to attain a high molecular weight or three dimensional crosslinking.